Photochemical Acid Generators

Various PAGs have been synthesized specifically for use in chemical amplification resists, reflecting their important impact on lithographic performance (Fig. 7) [13,30]. The choice of PAG depends on a number of factors such as the nature of radiation, quantum efficiency of acid generation, solubility, miscibility with resin, thermal and hydrolytic stability, plasticization effect, toxicity, strength and size of generated acid, impact on dissolution rates, cost, etc. In 

Because of the health hazard associated with fluorinated sulfonates, which was discovered recently, a new class of ionic acid generators has been proposed from 3M (imides and methides in Fig. 8) [56]. 

Photochemically generated acid must diffuse in resist film to catalyze desired reactions and to provide a gain mechanism for amplification. However, excessive diffusion (into the unexposed areas) destroys the linewidth control and eventually the resolution. Thus, as the minimum feature size becomes smaller and smaller, the control of acid diffusion plays a more important and difficult role. Therefore, investigation of acid diffusion in chemical amplification resist film is one of the most active areas of research today. A number of experimental procedures to measure acid diffusion length have been reported [67-88]  

Distribution of PAG in spin-cast resist film can affect image profiles. Thus, efforts have been made to tailor the PAG structure for adequate polarity and to 

Use of alkylsulfonium iodides in conjunction with PAG has been reported to enhance contrast and to control acid diffusion in e-beam imaging [92]. 

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Examples of such photochemical acid generators are shown in Chart 3.2. These onium salts, which are cationic photoinitiators originally developed for curing of epoxy resins (i09), can be used to formulate cross-linking negative resist materials (JOS), are very sensitive to electron beam and X-ray (JOS, 107, 108) radiation, and can be sensitized to longer wavelength radiation (JOS, 110, 111). 

In 1979, Frechet and Willson put forward a very productive idea of a chemical amplification that was used in the development of a new generation of photoresists.They decided to use a photoresist comprising of a photochemical acid generator (PAG) and a polymer that was able to switch from hydrophobic to hydrophilic in the course of acid catalyzed hydrolysis. The PAG reacts with light to produce an acid catalyst. During a subsequent postexposure bake, the catalyst diffuses and reacts with the polymer component, causing many reaction events in the polymer and recovers the acid catalyst. The acid molecules catalyze the deprotection reaction and provide a prerequisite for chemical amplification. The number of the reaction events initiated by single quantum absorption has been estimated to be of order of 100. ... 

National Science Foundation Optical density Postapply bake Photoactive compound Photochemical acid generator Positron annihilation spectroscopy Poly(fert-butoxycarbonyloxystyrene)... 

Fig. 5. Polynucleotide array fabrication process using photo-acid generator. In this process, acid is generated photochemically in a polymer film to effect fight-directed surface activation...

Scheme 1 Acid-catalyzed deprotection reaction. In the postexposure bake, the acid generated by the photoacid generator molecules (PAG) in a photochemical reaction catalyzes a deprotection reaction that cleaves the pendant group of the insoluble polymer, resulting in a polymer that is soluble in the developer...

The deprotection chemistry has been incorporated into the acid generator structure itself [177]. Phenolic hydroxyl groups pendant from triphenylsulfonium salts were protected with tBOC (Fig. 43). This dissolution inhibiting PAG mixed with PHOST becomes base soluble through photochemically-induced acid-catalyzed deprotection and thus the exposed area dissolves rapidly in aqueous base, which was named SUCCESS and promoted by BASF. A similar approach has been later reported on o-nitrobenzyl sulfonate acid generators, in which a tert-butyl ester was attached to the benzene ring for acid-generation and acid-catalyzed deprotection on one molecule (Fig. 43) [178]. 

In general, the chemical transformations associated with the chemical amplification mechanism in resists is effected through heating the exposed resist film, in a process called postexposure bake (PEB). Although, in principle, the active catalytic species (ions or radicals) could be generated from either photochemical (or radiochemical) acid or base generators, the acid generators are now used almost exclusively in advanced resist systems. ... 

The use of a photolabile acid generator to deprotect imagewise a functional group or depolymerize a polymer is the best-documented example of photochemical change systems that are known to proceed by a chain reaction mechanism. A new term, chemical amplification, has been coined to describe any of several such systems. Many photolabile acid generators are possible, but the most widely used have heen iodonium and sulfonium salts, which have their natural sensitivity between 200 and 300 nm hut can he substituted and sensitized out into the visible. 

Resist Chemistry. The basic chemistry of epoxy novolac based chemically amplified resists has been proposed in the past by Stewart et al. (9J. According to this the Bronsted acid generated either photochemically or through electron beam exposure from the onium salt induces acid catalysed polymerization of the epoxy functionality. This mechanism implies that the proton generated by the exposure is actually bound to the polymer. Since the lithography consequences of this mechanism are obvious we decided to seek possible experimental evidence for the proton binding in the resist film under conditions of lithographic interest. 

However, FT-IR spectrum of the PMOBH baked at 320 "C showed the formation of anhydride. If anhydride was formed, the weight loss should be 68%. This disagreement may be due to the non-volatile epoxy oligomers thermally formed from MOBH moiety. The PMOBH film containing p-toluenesulfonic acid generated photochemically started to decompose at ca. 150 C. The weight loss for PMOBH films with and without acids was almost the same. The crosslinked PMOBH film could be thermally decomposed to poly(methacrylic acid) and/or poly(methacrylic acid) anhydride at lower temperatures than PMOBH in ttie absence of acid. 

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